Project Summary Mild traumatic brain injury is the most common form of impact and is prevalent among contact sports athletes and military personnel. Repetitive mild traumatic brain injury (rmTBI) can generate several molecular cascades that lead to neuropathological consequences in the brain. Moreover, rmTBI has been correlated to neurodegenerative diseases such as chronic traumatic encephalopathy (CTE). CTE is a slow, progressive tauopathy that exhibits neurofibrillary tangles (NFTs) in brain accompanied by psychiatric and cognitive manifestations. NFTs are aggregates formed from the misfolding and aggregation process of tau protein and are canonical in tauopathies such as CTE and Alzheimer?s disease (AD), among others. Individuals following a TBI event can demonstrate early tau accumulation, independent of age, suggesting a potential pathological link between TBI and disease. Formation of amyloids, such as NFTs from tau, is proposed to follow a nucleation-polymerization model where a misfolded seed can trigger native proteins to misfold and aggregate. Introducing a pre-formed seed can accelerate this reaction. The development of tau inclusions and effect on clinical symptoms in connection to rmTBI remain to be elucidated; moreover, effective diagnostic methods for TBI and tau pathology are lacking. This proposal aims to determine the effect of rmTBI on tau pathology along with psychiatric and cognitive sequelae and exploit this misfolded tau for diagnostic assessment in brain and biological fluids by a novel, sensitive assay called tau-protein misfolding cyclic amplification (PMCA). These experiments intend to explore the role of rmTBI, mimicking sub-concussive impacts in athletes, in vivo in contributing to tau pathology and determine if early formed rmTBI-induced tau aggregates have seeding capabilities. Tau-PMCA will be utilized to detect the initiation of tau seeds in brain and biological fluids, such as cerebrospinal fluid (CSF) and blood plasma, triggered by rmTBI in mice as well as connecting the neurobehavioral changes over time. Finally, in a translational approach, tau-PMCA will be used to determine the formation of the first tau seeds in CSF and blood plasma from TBI and AD cases as a diagnostic tool. This proposal conveys the mission of the NIH by advancing science and advocating the establishment of a promising diagnostic strategy. Overall, this proposal will ameliorate the understanding of rmTBI in relation to tau pathology and behavior and potentially detecting disease onset for therapeutic strategies in the future.